Ultrasonic processes and circuits for performing them

ABSTRACT

A process for selective graphic representation and/or evaluation of the Doppler spectrum of objects limitedly resistant to sonic intensity, for example biological organs and tissues, by an ultrasonic process wherein a material is introduced in the examination area to be acoustically irradiated, nonlinear oscillations are produced in the examination area by irradiated ultrasonic waves and the signal is evaluated by an ultrasonic converter. Also, a circuit for carrying out the above process is disclosed.

This is a continuation of the application Ser. No. 08/401,444 filed Mar.9, 1995, now abandoned which is a continuation of Ser. No. 08/076,221,filed Jun. 14, 1993, now U.S. Pat. No. 5,410,516, issued Apr. 25, 1995,which is a continuation of Ser. No. 07/684,900, filed Apr. 26, 1991, nowabandoned, which is which is based on PCT/DE89/00560, filed Aug. 23,1989.

The invention relates to ultrasonic processes according to theintroductory clause of claims 1 or 2 and circuits for performing them.

In ultrasonic technology, ultrasonic waves are irradiated in anexamination area for selective graphic representation and/or evaluationof the Doppler spectrum. Combined transceiver sound heads are usuallyused in the processes and equipment for material testing and forexamination of biological tissues. In this way, a sound frequency(f_(o)), which is the same for the sending and receiving, is determinedby the crystals of the oscillator and the equipment electronics. Atypical 5 MHz sound head has a frequency range of about 3-7 MHz with amaximum at f_(o) =5 MHz. In the same frequency range, the reflectedand/or backscattered signal is received with the pulse-echo process.Such equipment and processes are also used in the examination ofbiological tissue with the use of ultrasonic contrast media. Signalportions lying outside the specified frequency range, such as, forexample, oscillations being in a harmonic ratio to the sendingfrequency, are not used for the graphic representation of theexamination object and other analyses, such as, e.g., Dopplermeasurements. In the previously known processes and equipment systems,several sound heads, which are changed during the examination, arefurther used to cover a sizable frequency range.

Improving the image quality by using harmonic multiples of theexcitation frequency in ultrasonic microscopy is known from thebibliographical reference L. Germain, J. O. N. Cheeke, (J. Accoust. Soc.Am. E3 (1988) 942). For this purpose, however, ultrasonic waves withvery high amplitude have to be irradiated to produce nonlinearoscillations in the path in the examination area, and by thisnonlinearity, a transmission of energy from the oscillations with thebasic frequency takes place in higher harmonic oscillations.

But such a process cannot be used in the ultrasonic examination with lowfrequencies, for example, in the range of 1-10 mHz of objects which arenot resistant to high sound intensities, such as, especially, biologicaltissues.

The object of the invention is to broaden the range of use of ultrasonicprocesses for objects, limitedly resistant to sound intensity,especially biological tissues, for selective graphic representation andevaluation of the Doppler spectrum and to provide circuits forperforming these processes.

According to the invention, this object is achieved in that a materialis introduced in the examination area to be acoustically irradiated,with which nonlinear oscillations in this area are produced byirradiated ultrasonic waves, a broad-band, acoustically highly damped,electrically matched ultrasonic converter with one or more controllableconverter elements assembled individually or in groups, whichcorresponds to a frequency band which, in addition to the excitationfrequency, comprises at least α/2 and/or α/3 and/or α/4 times excitationfrequency (f_(o)), with α=whole, number, is excited for acousticirradiations of the examination area and the excitation frequency and/orat least one of α/2, α/3, α/4 times it are evaluated from the ultrasonicsignal received by the ultrasonic converter, reflected from theexamination area or backscattered from the latter.

If α is an even-numbered multiple of the denominator, the correspondingoscillations are the harmonics. If α< the denominator, theseoscillations are called subharmonics in the literature. If α>thedenominator, ultraharmonic oscillations are involved.

By introducing materials or media in the examination area to beacoustically irradiated, which produce a nonlinearity, it issurprisingly possible, even in low sonic intensities, which are notharmful, to obtain intensive and strongly frequency-shifted stray and/ortransmission signals in addition to excitation frequency f_(o). Thesestray and/or transmission signals are intensive particularly inharmonics (2 f_(o), 3 f_(o) . . . ), subharmonics (1/2 f_(o), 1/3 f_(o),3/4 f_(o)) and ultraharmonics (3/2 f_(o), 5/4 f_(o). . . ) of excitationfrequency. With this process, irradiation can be performed with lowfrequencies, so that a greater penetration depth is obtained andreceiving signals of higher frequencies can be evaluated.

In an advantageous way, a selective evaluation of the signal portionsinfluenced by the fed materials or media as well as a selective displayof the areas filled with these media is possible, without a previouslynecessary subtraction being made between two or more states recordedbefore and after the application of the materials or media. Especially,the Doppler effect caused can be evaluated independently of artifacts.

Nonlinear stray elements are advantageously introduced in theexamination area. But in the examination area, a nonlinear ultrasoniccontrast medium can also be introduced in the form of a solution orsuspension and especially microbubbles or agents producing microbubbles.

The introduction of a microbubble suspension with a concentration of10⁻³ % by weight to 30% by weight of dry substance in a suspensionmedium leads to good results, and, surprisingly, the low lower limit of10⁻³ % by weight is attained with the process according to the inventionand the circuit according to the invention.

In the process according to the invention, the sonic converter isadvantageously excited by a function generator, with which HF burstswith adjustable amplitude and adjustable center frequency (f_(T)) areproduced in the range of 0.3 MHz to 22 MHz, preferably 1 MHz to 11 MHz,and with 0.5 to 20, preferably 1-5, periods. In this case, it has beenshown as especially advantageous to evaluate frequencies which aresmaller than sonic converter (transmitter) center frequency f_(T).

In the evaluation, it is advantageous, by a computer-controlled gatecircuit, to select at least one period and to determine the relatedfrequency spectrum in an analog or digital manner. In this case, thetime window length and the number of periods per burst are adjustedbetween the optimum frequency resolution and optimum high-sensitivityresolution.

With the process according to the invention, Doppler effects canadvantageously be evaluated in harmonics of the excitation frequency andin the mixed products, such as, the upper sideband in 2-frequencyexcitations. This permits the representation of slower flows withoutdisturbances by movements of the vessel wall.

An improved penetration depth and/or space resolution, which is veryadvantageous in graphic representation and in Doppler measurements,further results in the evaluation of harmonic signal portions or signalsin the upper sideband.

The circuit, according to the invention, for performing the processdescribed above exhibits a function generator whose output is connectedwith the oscillator of an acoustically highly damped, electricallymatched, broadband converter element by an T/R (transceiver) circuitsynchronized by the function generator, which is downstream from asignal processing system.

In another embodiment of the circuit, the function generator isconnected to the input of a converter whose output is connected to asignal processing system.

In the first case mentioned, the burst produced by the functiongenerator in the "sending" circuit of the T/R switch is transmitted tothe oscillator of the converter and the signal received by the converteris transmitted in the switched-on "receiving" position of the T/Rcircuit to the evaluating system. In the second case, the input andoutput are separated in the converter so that an T/R switch is notnecessary.

A converter element, whose center frequency f_(T) is greater than theupper limit of the operating range, is used with special advantage. Thisconverter element is designed so that, as a function of the frequency inthe frequency range below excitation or center frequency f_(T), thesonic intensity radiated by the converter element exhibits a positivefirst derivative with respect to the frequency, which is approximatelyconstant especially in the operating range, or so that the sonicintensity which is in the operating range itself has a constant value.By this almost rectilinear frequency response in the operating range fora similar frequency response, especially the damping can be largelybalanced, in the irradiated examination area. By this circuit and theconverter used, a change of the frequency used for examination ispossible without a change in sound heads. Further, in the evaluation ofspectra for material characterization, especially in the tissuecharacterization, the respectively optimum ratio of space resolution andfrequency resolution can be selected.

The process according to the invention can advantageously be performedby a circuit, which exhibits a multielement converter withphase-delayed, actuated converter elements, to perform a phase-array ora dynamically focused process. In this circuit, the output of a functiongenerator is connected by an n-path signal divider, ncomputer-controlled delay circuits and n T/R switches, controlled by thefunction generator or a computer, to the inputs of n acoustically highlydamped, electrically matched, broadband converter elements, whoseoutputs are connected by n T/R switches each to an m-path signaldivider. These m-path signal dividers are each connected by m delaycircuits and m fixed or variable circuits for frequency band selectionand further by a circuit for in-phase summation and optionally signaldistribution to a system for selective further processing of m frequencybands.

In another achievement of the object of the invention, a material isintroduced in the examination area to be acoustically irradiated withwhich nonlinear oscillations are produced in this area by irradiatedultrasonic waves, a broadband, acoustically highly damped, electricallymatched ultrasonic converter with one or more controllable converterelements individually or assembled in groups is excited by two HFbursts, whose respective excitation frequencies are different from oneanother, and are smaller than half the frequency upper limit of theoperating, range, and signal combinations of both excitationfrequencies, especially their sum or difference frequency, are evaluatedfrom the ultrasonic signals received from the ultrasonic converter,reflected from the examination area or backscattered from the latter.

In this case, a stronger receiving signal with a frequency of thecombination of the frequencies of the irradiated signals, especially thesum or difference frequency, is obtained by irradiation of two signalsseparated from one another. In this case, the sum frequency isespecially of interest because of the achievably higher spaceresolution. In this process, a converter element can be excited by twoHF bursts. But the possibility also exists of exciting two separateconverter elements respectively with one HF burst, and the centerfrequencies of these HF bursts vary and are smaller than half the upperlimit of the frequency of the operating range.

By the nonlinearity produced according to the invention, a strongerreceiving signal at f_(o) +f_(p), i.e., at about 4 MHz, is obtained, forexample, with two low-frequency signals, e.g., f_(o) ≈f_(p) ≈2 MHz, thanif only one sending signal of frequencies f_(o) +f_(p) is used with sametotal output I_(o), I_(p). This phenomenon makes possible a higherpenetration depth at high observation frequencies.

As materials or media, which produce the nonlinearity, the same can beused as in the process for evaluating the harmonic frequencies of theexcitation frequency. Basically, the same circuit elements can be usedwith addition of a second HF generator.

In the circuit with a multielement converter, the second signal isalways sent only in the respective direction of the first signal toreduce the average output irradiated in the examination area and isbegun about 1 to 2 periods earlier and lasts until the end of the firstburst signal. To achieve this, the second signal from the secondgenerator is influenced by corresponding delay circuits so that itreaches the same converter elements in the sound head after passage ofthe T/R switch and is radiated in the same direction as the firstsending signal. The circuit matrix then receives signals in thefrequency sum. In this case, the T/R switch is controlled by thelonger-lasting second sending signal.

Embodiments of the invention are to be explained in the followingdescription with reference to the figures of the drawings.

There are shown in:

FIG. 1, a block diagram,

FIG. 2, a diagrammatic sectional representation of a test vessel,

FIG. 3, a representation of the acoustic power curve of the converter asa function of the frequency,

FIG. 4-9, graphic representations of the backscatter spectrum and

FIG. 10, another block diagram.

For the production of the signals provided for further processing,represented in FIG. 4-9, the circuit represented in FIG. 1 is usedtogether with the test vessel represented in FIG. 2, and the broadbandsound head exhibits the power characteristic represented in FIG. 3.

Periodically repeated, electric sending pulses--HF bursts--of variablefrequency f_(o) in operating range f_(o) min. . . f_(omax) (f_(omin)=0.3 MHz<f_(o) <f_(omax) =22 MHz) and variable bandwidth, given bynumber n of sine periods per burst: 0.5<n<20 with adjustable amplitude,are produced by a function generator 1, which is controlled by centralcomputer 15. This central computer 15 controls both the course of themeasurement and its evaluation. Output 2 of generator 1 leads to asending-receiving switch 3, which, as diagrammatically represented, issynchronized by generator 1. T/R switch 3 can also be controlleddirectly by computer 15. Output 2 of T/R switch 3 is connected to abroadband, matched and focused converter element 4. The special featuresof this converter element 4 are diagrammatically represented in FIG. 3.This converter exhibits a large wideband without disturbing resonancesin the operating range, further, a good electrical and acousticimpedance matching and a transmitter center frequency f_(T) >f_(o) max.In the described example, f_(t) =17 MHz. This converter can also exhibitspatially and electrically separated sending and receiving converterelements. In this case, T/R switch 3 is unnecessary. In addition,another converter element for sending a second, independenthigh-frequency signal can be advantageously provided.

The signal received from converter element 4 is fed by the reversed T/Rswitch to a broadband input amplifier 16, which, in digital frequencyanalysis, is downstream from an antialiasing filter 17. Broadband inputamplifier 16 exhibits a bandwidth>f_(o) max. Filter 17 has, for example,a cutoff frequency of 10 MHz. A rapid A/D converter, in which the signalis digitalized, for example with a Nyquist frequency of 12.5 MHz, isdownstream from filter 17. The further processing of the signals takesplace in a digital storage oscilloscope and in the central computer.Plotter 19 is downstream from AID converter 18.

FIG. I shows that the AID converter is triggered by function generator1.

The digitalized signal is stored and further processed in a way known inthe art. It is available especially for necessary corrections. Beforethe A/D conversion, a signal, which is digitalized only after analogfurther processing, can also be branched off.

FIG. 2 diagrammatically shows the geometry of test vessel 20, with whichthe measuring results represented below are achieved.

As FIG. 2 shows, sound head 4 is placed in test vessel 20. It is a 17MHz sound head, which is broadband, matched and focused. Water is intest vessel 20. Two sheets 21 separate a test area, in which 10 mg ofultrasonic contrast medium is dissolved in 3 ml of H₂ O.

The signals reflected and/or backscattered in the measuring area betweensheets 21 contain special portions which were obtained by an interactionbetween the sending pulse (at f_(o)) and the nonlinear contrast mediumintroduced in the measuring object.

FIG. 3 diagrammatically shows the frequency band of the converterelement in the sound head. It can be seen that in the operating range,the frequency response of the oscillator in the sound head is almostlinear. The frequency response in the operating range can be used tobalance a similar frequency response in the test piece. But thefrequency response in the test piece can also be corrected later by aweighting.

In measuring, an advantageous period is selected in the time range by acomputer-controlled gate circuit, not shown. Several periods can also beselected. The related spectrum is calculated by an FFT circuit (FastFourier Transformation) and examples of such spectra are represented inFIG. 4-9. By the selection of a corresponding time window length, achoice can be made between optimum frequency resolution and optimumhigh-sensitivity resolution. In FIG. 4-8, the spectrum in each case isrepresented by the time window. To bring out clearly the spectralcomponents in these fig., a long time window, i.e., a poorhigh-sensitivity resolution, was selected. FIG. 4 illustrates the timecharacteristic of the sending pulse after a reflection on the excitingwindow without contrast medium, f_(o) =4.0 MHz, +15 dBm on the soundhead. A clear signal can be detected at 4 MHz. The signal represented inthe upper part of FIG. 4 is an averaged output spectrum, which wasobtained behind the low-pass filter with a Nyquist frequency of 50 MHz.

In FIG. 5, the backscatter signal from the test chamber is representedwithout an ultrasonic contrast medium. FIG. 6 shows the backscattersignal seven minutes after adding 10 mg of contrast medium in 3 ml of H₂O. A clear peak can be detected at 2×f_(o).

FIG. 7 shows a measurement after 21 minutes under the conditionsrepresented in FIG. 5. The operation was performed with a frequencyf_(o) =3 MHz. The received spectrum clearly shows the first and secondharmonics at 6.0 and 9.0 MHz. FIG. 6 illustrates the backscatter signal15 minutes after adding an ultrasonic contrast medium of smallerconcentration. The operation was performed on the sound head with afrequency, f_(o), of 4 MHz+20 dBm. The spectrum represented in the upperpart of FIG. 8 shows, with higher frequency resolution, subharmonics at1/2 f_(o), ultraharmonics at 3/2 f_(o) and the first harmonics at 2f_(o).

FIG. 9 shows a backscatter signal of linear ultrasonic contrast mediumf_(o) =4 HMz+15 dBm on the sound head. The spectrum shows only abackscattering in the excitation frequency.

It can be seen that the represented spectra exhibit clear amplitudes inthe frequency ranges which do not occur in the sending spectrum if aninteraction has taken place with a nonlinear contrast medium. Spectralchanges can be evaluated, which are caused by a Doppler effect. To usethe circuit used in the described embodiments for the imaging ultrasonicprocesses, additional components are provided, if a sound head of"phased array" type or a dynamically focused sound head is used. Such ablock diagram is illustrated in FIG. 10.

The sending signal of function generator 1 (frequency f_(o)) is fed fromoutput 2 to n path signal divider 5. The distribution takes place withone branch each per converter element. In the represented embodiment, nconverter elements 4 are provided. Actuation of these converter elements4.1 . . . 4.n takes place by delay circuits 7.1 . . . 7.n and T/Rswitches 3.1 . . . 3.n controlled by the generator or computer. Thedelay periods are set for each converter element by the computer so thatthe desired directional characteristic on the sound head results in theselected sending frequency. The same directional characteristic is setby the computer by corresponding delays in the receiving part. Thesignal received from sound heads 4.1 . . . 4.n is fed by T/R switches3.1 to broadband input amplifier 6.1, 6.n. Each input amplifier 6.1, . .. , 6.n actuates an m-path signal divider 10, to which correspondinglycontrolled or set delay circuits 11 are placed downstream in each case,which supplies circuits 12 for frequency band selection. Circuits forin-phase summation of the frequency bands and for optional signaldistribution are downstream. A selective further processing of theindividual frequency bands with the process known in the art follows.

Especially, an evaluation of the frequencies, which are not identicalwith f_(o), for example, 1/2 f_(o), 2 f_(o), takes place.

The delay circuits can be variable or fixed. The distribution of thereceived signals to the m-path signal dividers produces the desirednumber of frequency bands, whose position and width are set with bandfilters. As an alternative, the distribution can also take place so thatthe receiving signal, with a subsidiary signal that is differentdepending on the frequency band and derived from a sending signal, ismixed so that the individual bands in the following stages can operatewith homogeneous components.

The frequency band around f_(o) yields the usual results, while theother bands contain strongly frequency-shifted and nonlinear signalportions from interactions of the sending signal with the nonlinearultrasonic contrast media.

The further processing steps and signal analyses could take place in anyfrequency channel or in several parallel frequency channels according tothe known processes.

To use two sending frequencies f_(o) and f_(p), the second generator,represented on the right side of FIG. 10, is provided, which isconnected by signal dividers and delay lines 15 to T/R switches 3.1 . .. 3.n. Second generator 1 makes it possible acoustically to irradiate atleast the space area in the test object, which is determined by theinstantaneous directional characteristic and the receiving gate. Thedesign can be such that in addition to the described broadband converterelements, at least one also broadband sending converter is located inthe sound head, which is preferably separated electrically from theothers and which is fed by second, independent sending generator 1. Butboth sending signals can also be superposed electrically so that thesame converter elements can be used.

We claim:
 1. Ultrasonic process, useful for objects limitedly resistantto sonic energy for selective graphic representation and/or evaluationof the Doppler spectrum, in whicha material is introduced in anexamination area to be acoustically irradiated with which nonlinearoscillations in this area are produced by irradiated ultrasonic waves, abroadband, acoustically highly damped, electrically matched ultrasonicconverter with one or more controllable converter elements individuallyor assembled in groups, which responds to a frequency band which, inaddition to the excitation frequency, comprises at least α/2 and/or α/3and/or α/4 times excitation frequency (f_(o)), with α=whole number, isexcited to acoustically irradiate the examination area, and theexcitation frequency and/or at least one of α/2, α/3, α/4 times it areevaluated from the ultrasonic signal received from the ultrasonicconverter, reflected from the examination area or backscattered from thelatter.
 2. Ultrasonic process, useful for objects limitedly resistant tosonic energy, for selective graphic representation and/or evaluation ofthe Doppler spectrum, in whicha material is introduced in an examinationarea to be acoustically irradiated with which nonlinear oscillations inthis area are produced by irradiated ultrasonic waves, a broadband,acoustically highly damped, electrically matched ultrasonic converterwith one or more controllable converter elements individually orassembled in groups, is excited by two HF bursts, whose respectiveexcitation frequencies are different from one another, and are smallerthan half the frequency upper limit of the operating range, and signalcombinations of both excitation frequencies, are evaluated from theultrasonic signals received from the ultrasonic converter, reflectedfrom the examination area or backscattered from the latter. 3.Ultrasonic process according to claim 1, wherein the material containsstray elements producing nonlinear oscillations.
 4. Ultrasonic processaccording to claim 1, wherein the material is an ultrasonic contrastmedium in the form of a solution, emulsion or suspension.
 5. Ultrasonicprocess according to claim 1, wherein microbubbles or agents producingmicrobubbles are the material.
 6. Ultrasonic process according to claim4, wherein a microbubble suspension with a concentration of 10⁻³ % byweight to 30% by weight of dry substance is introduced in the suspensionmedium.
 7. Ultrasonic process according to claim 1, wherein theultrasonic converter is excited by at least one function generator, withwhich HF bursts of adjustable amplitude and adjustable excitationfrequency (f_(o)) are produced in the range of 0.3 MHz to 22 MHz, andwith 0.5 to 20 periods.
 8. Ultrasonic process according to claim 1,wherein frequencies that are smaller than ultrasonic converter centerfrequency (f_(T)) are evaluated.
 9. Ultrasonic process according toclaim 1, wherein at least one period is selected and the relatedfrequency spectrum is determined in an analog or digital manner in theevaluation by a computer-controlled gate circuit.
 10. Ultrasonic processaccording to claim 9, wherein time window length and the number ofperiods per burst is adjusted depending on the desired frequencyresolution and high-sensitivity resolution.
 11. Circuit for performingthe process according to claim 1, characterized by1. a functiongenerator (1), whose output (2) is connected by
 2. an T/R switch (3),synchronized by function generator (1), from which a signal processingsystem is downstream,
 3. is connected to an oscillator of anacoustically highly damped, electrically matched, broadband ultrasonicconverter element (4).
 12. Circuit for performing the process accordingto claim 1, characterized by1. a function generator (1), whose output(2) is connected to
 2. the input of the ultrasonic converter,
 3. whoseoutput is connected to a signal processing system.
 13. Circuit accordingto claim 11, wherein the sonic energy radiated by the ultrasonicconverter exhibits, as a function of the frequency in the frequencyrange below ultrasonic converter center frequency (f_(T)), a positivefirst derivative with respect to the frequency, which is constant inoperating range (f_(o) min <f_(o) <f_(o) max <f_(T)) or wherein thissonic intensity has a constant value in this frequency range. 14.Circuit for performing the process according to claim 1 with amultielement ultrasonic converter with n-phase-delayed, actuatedultrasonic converter elements, characterized by1. a function generator(1), whose output (2) is connected
 2. by an n-path signal divider (5),3. n computer-controlled delay circuits (7.1.1 . . . 7.n.1)
 4. and n T/Rswitches (3.1.1 . . . 3.n.1) controlled by function generator (1) or acomputer, to
 5. the inputs of the ultrasonic converter elements, whose6.outputs are connected by n T/R switches (3.1.1 . . . 3.n.1) to, in eachcase, an
 7. m-path signal divider (10),
 8. which are connectedrespectively by m delay circuits (11),
 9. m fixed or variable circuits(12) for frequency band selection and
 10. a circuit for in-phasesummation and optional signal distribution
 11. to a system for selectivefurther processing--individually or parallel--of m frequency bands. 15.Ultrasonic process according to claim 2, whereinthe two HF bursts areproduced by two function generators and are fed either to an ultrasonicconverter element or to two ultrasonic converter elements, and whereinwith each function generator, HF bursts of adjustable amplitude andadjustable center frequency (f_(o)) are produced in the range of 0.5 to20 MHz, periods.
 16. A system for obtaining an image of biologicaltissue comprising biological tissue, and in operative associationtherewith,function generator means for generating excitation ultrasonicenergy at a frequency of from 0.3 MHZ to 22 MHZ for application to theexamination area; transducer means for transmitting the ultrasonicenergy to the examination area and for detecting reflected and/orbackscattered energy from the examination area; processing means forevaluating from the reflected and/or backscattered ultrasonic energyfrom the examination area at least one harmonic of the excitationfrequency and for generating an image of biological tissue in theexamination area.
 17. A system for obtaining an image of biologicaltissue comprising biological tissue, and in operative associationtherewith,function generator means for generating excitation ultrasonicenergy of two different frequencies both of from 0.3 MHZ to 22 MHZ forsimultaneous application to the examination area; transducer means fortransmitting the ultrasonic energy at both frequencies to theexamination area and for detecting reflected and/or backscattered energyfrom the examination area; processing means for evaluating from thereflected ultrasonic energy from the examination area at least one ofthe sum or difference of the two excitation frequencies and forgenerating an image of biological tissue in the examination area.
 18. Aprocess for imaging biological tissue in an examination area containingan ultrasonic contrast agent, which comprises:subjecting the examinationarea to ultrasonic energy of frequency, f_(o), and imaging theexamination area using a harmonic, subharmonic or ultraharmonic offrequency, f_(o).
 19. A process for imaging biological tissue in anexamination area containing an ultrasonic contrast agent, whichcomprises:subjecting the examination area to ultrasonic energy offrequency, f_(o), and ultrasonic energy of a different frequency, f_(p),and imaging the examination area using the sum or difference of theexcitation frequencies f_(o) and f_(p).
 20. A method for imaging apatient to whom an ultrasonic contrast agent has been administered,which comprises:irradiating an area of the patient to be imaged withultrasonic energy of frequency, f_(o), and imaging the area using aharmonic, subharmonic or ultraharmonic of frequency, f_(o).
 21. A methodfor imaging a patient to whom an ultrasonic contrast agent has beenadministered, which comprises:irradiating an area of the patient to beimaged with frequency, f_(o), and ultrasonic energy of a differentfrequency, f_(p), and imaging the area using the sum or difference ofthe excitation frequencies f_(o) and f_(p).
 22. The process of claim 19,wherein the frequency, f_(o), is of 0.3-22 MHZ with 0.5 to 20 periods.23. The process according to claim 19, wherein the ultrasonic contrastagent is a solution, an emulsion or a suspension.
 24. The processaccording to claim 23, wherein the contrast agent is a microbubblesuspension having a concentration of from 10⁻³ % by weight to 30% byweight dry substance in the suspension.
 25. The process of claim 19wherein the two HF bursts are generated by two function generators andfed either to one ultrasonic transducer element or to two ultrasonictransducer elements with adequate delay to guarantee simultaneoustransmission in the same direction, and wherein each function generatorgenerates HF bursts of adjustable amplitude and adjustable excitationfrequency, f_(o) or fρ, of 0.5 to 20 MHZ with from 1 to 25 cycles. 26.The process of claim 25, wherein the excitation frequency, f_(o) or fρis 1 to 5 MHZ and 1 to 10 cycles are generated.
 27. The process of claim19, wherein the reflected and backscattered signal is processed withcomputer-controlled gate circuit, at least one time window beingselected and the associated frequency spectrum being determined inanalog or digital manner.
 28. The ultrasonic process of claim 27,wherein the length of the time window and the number of cycles per burstare adjusted according to a selected frequency resolution and spatialresolution.
 29. An apparatus for ultrasonic harmonic imaging ofbiological tissue in an examination area comprising:function generatormeans for generating excitation ultrasonic energy at a frequency of from0.3 MHZ to 22 MHZ for application to the examination area; transducermeans for transmitting the ultrasonic energy to the examination area andfor detecting reflected and/or backscattered energy from the examinationarea; processing means for evaluating from the reflected and/orbackscattered ultrasonic energy from the examination area at least oneharmonic of the excitation frequency and for generating an image ofbiological tissue in the examination area.
 30. The apparatus of claim29, wherein function generator generates said excitation ultrasonicenergy characterized by variable amplitude and period.
 31. An apparatusfor ultrasonic harmonic imaging of biological tissue in an examinationarea comprising:function generator means for generating excitationultrasonic energy of two different frequencies both of from 0.3 MHZ to22 MHZ for simultaneous application to the examination area; transducermeans for transmitting the ultrasonic energy at both frequencies to theexamination area and for detecting reflected and/or backscattered energyfrom the examination area; processing means for evaluating from thereflected ultrasonic energy from the examination area at least one ofthe sum or difference of the two excitation frequencies and forgenerating an image of biological tissue in the examination area. 32.The apparatus of claim 31, wherein the processing means is forevaluating the sum of the two excitation frequencies.
 33. The apparatusof claim 31, wherein function generator generates said excitationultrasonic energy characterized by variable amplitude and period.
 34. Anapparatus for ultrasonic harmonic imaging of biological tissue in anexamination area comprising:an HF burst generator for generating a burstof ultrasonic energy at an excitation frequency; a computer connected tosaid HF burst generator for controlling said HF burst generator and fordata processing; a T/R switch, having a sending position and a receivingposition, connected to said HF burst generator; one or more transducersconnected to said T/R switch for transmitting bursts at the excitationfrequency and for receiving ultrasonic signal reflected and/orbackscattered from the examination area; a wide-band pre-amplifierconnected to said T/R switch for receiving the reflected and/orbackscattered ultrasonic signal; an anti-aliasing filter connected tosaid wide-band pre-amplifier for filtering the received signal such thatat least one harmonic of the excitation frequency can be evaluated; anA/D converter connected to said anti-aliasing filter for receiving thefiltered signal and for digitizing the signal, wherein said A/Dconverter is also connected to said computer to provide the digitizedsignal to said computer for processing an image of the inspection zone.35. An ultrasonic harmonic imaging apparatus as in claim 34, whereinsaid HF burst generator generates bursts of excitation frequency from0.3 MHZ to 22 MHZ.
 36. An ultrasonic harmonic imaging apparatus as inclaim 34, wherein said bursts of excitation frequency are characterizedby adjustable amplitude and period of 0.5 to 20 cycles.
 37. An apparatusfor ultrasonic harmonic imaging of biological tissue in an examinationarea comprising:HF burst generators for generating one burst ofultrasonic energy at two different frequencies with adequate delay toguarantee simultaneous transmission in the same direction; a computerconnected to said HF burst generators for controlling said HF burstgenerators and for data processing; a T/R switch, having a sendingposition and a receiving position, connected to said HF burstgenerators; one or more transducers connected to said T/R switch fortransmitting bursts at the two different transmit frequencies and forreceiving ultrasonic signal reflected and/or backscattered from theexamination area; a wide-band pre-amplifier connected to said T/R switchfor receiving the reflected and/or backscattered ultrasonic signal; ananti-aliasing filter connected to said wide-band pre-amplifier forfiltering the received signal such that at least one of the sum ordifference of the two transmit frequencies can be evaluated; an A/Dconverter connected to said anti-aliasing filter for receiving thefiltered signal and for digitizing the signal, wherein said A/Dconverter is also connected to said computer to provide the digitizedsignal to said computer for processing an image of the examination area.38. The apparatus of claim 37, wherein the anti-aliasing filterconnected to said wide-band pre-amplifier is for filtering the receivedsignal such that the sum of the two transmit frequencies can beevaluated.
 39. An ultrasonic harmonic imaging apparatus as in claim 37,wherein said HF burst generators generate bursts of excitation frequencyfrom 0.3 MHZ to 22 MHZ.
 40. An ultrasonic harmonic imaging apparatus asin claim 37, wherein said bursts of excitation frequency arecharacterized by adjustable amplitude and period of 0.5 to 20 cycles.41. The process of claim 18, wherein the ultrasonic contrast agentcontains microbubbles or produces microbubbles upon exposure toultrasonic energy.
 42. The process of claim 18, wherein the ultrasonicenergy of frequency, f_(o), is generated by applying an HF burst ofexcitation frequency, f_(o), to electrically excite a wideband,acoustically highly damped, electrically matched ultrasonic transducerhaving one or more transducer elements controllable individually or ingroups.
 43. The process of claim 18, wherein the frequency, f_(o), is of0.3-22 MHZ with 0.5 to 20 periods.
 44. The process of claim 18, whereinthe frequency, f_(o), is of 1-22 MHZ.
 45. The process of claim 18,wherein, additionally the frequency, f_(o), from the reflected andbackscattered ultrasonic signal is used for imaging or evaluating theDoppler spectrum of the examination area.
 46. The process according toclaim 18, wherein the ultrasonic contrast agent is a solution, anemulsion or a suspension.
 47. The process according to claim 41, whereinthe contrast agent is a microbubble suspension having a concentration offrom 10⁻³ % by weight to 30% by weight dry substance in the suspension.48. The process of claim 18, wherein the frequency f_(o) is 1 MHZ to 11MHZ and 1 to 5 cycles are generated.
 49. The process of claim 18,wherein, from the reflected and backscattered ultrasonic signal,frequencies that are lower than the center frequency (f.sub.τ) of theultrasonic transducer are evaluated.
 50. The process of claim 18,wherein the reflected and backscattered ultrasonic signal is processedwith a computer-controlled gate circuit, at least one time window beingselected and the associated frequency spectrum being determined inanalog or digital manner.
 51. The process of claim 50, wherein thelength of the time window and the number of cycles per burst areadjusted according to a selected frequency resolution and spatialresolution.
 52. The process of claim 19, wherein the ultrasonic contrastagent contains microbubbles or produces microbubbles upon exposure toultrasonic energy.
 53. The process of claim 19, wherein the microbubbleshave nonlinear vibrations when exposed to ultrasonic energy.
 54. Theprocess of claim 19, wherein the ultrasonic energy of frequencies f_(o)and f_(p) are generated by applying two properly combined HF bursts ofexcitation frequencies f_(o) and f_(p) to electrically excite awide-band, acoustically highly damped, electrically matched ultrasonictransducer having one or more transducer elements controllableindividually or in groups.
 55. The process of claim 54, wherein thefrequencies f_(o) and f_(p) are each less than half the upper frequencylimit of the working range of the ultrasonic transducer.